Although austenitic stainless steels have hitherto been used in the component members of nuclear reactors, especially fast reactors which are required to have excellent high-temperature strength and resistance to neutron irradiation, they have limitations on irradiation resistance such as swelling resistance. On the other hand, martensitic stainless steels have the disadvantage of low high-temperature strength although they are excellent in irradiation resistance.
Therefore, oxide dispersion strengthened martensitic steels have been developed as materials that combine irradiation resistance and high-temperature strength and there have been proposed techniques for improving high-temperature strength by adding Ti to oxide dispersion strengthened martensitic steels, thereby finely dispersing oxide particles.
For example, Japanese Patent Laid-Open No. 5-18897/1993 discloses a tempered oxide dispersion strengthened martensitic steel which comprises, as expressed by % by weight, 0.05 to 0.25% C, not more than 0.1% Si, not more than 0.1% Mn, 8 to 12% Cr (12% being excluded), 0.1 to 4.0% in total of Mo+W, not more than 0.01% O (O in Y2O3 and TiO2 being excluded) with the balance being Fe and unavoidable impurities, and in which complex oxide particles comprising Y2O3 and TiO2 having an average particle diameter of not more than 1000 Å are homogeneously dispersed in the matrix in an amount of 0.1 to 1.0% in total of Y2O3+TiO2 and in the range of 0.5 to 2.0 of the molecular ratio TiO2/Y2O3.
However, even when oxide dispersion strengthened martensitic steels are produced by adjusting the total amount of Y2O3 and TiO2 and the ratio of these oxides and besides the total amount of Mo and W as disclosed in the Japanese Patent Laid-Open No. 5-18997/1993, there are cases where oxide particles are not finely dispersed in a homogeneous manner and it follows that in such cases the expected effect on an improvement in high-temperature strength cannot be achieved.